Electric range unit control mechanism embodying material of changeable character



July 16, 1940.

ELECTRIC RANGE UN IT CONTR MATERIAL 0F CHANGEABLE CHARACTER Filed June4, 1958 J W. MYERS 0L MECHANISM EMBODYING 4 Sheets-Sheet 1 J. W. MYERSJuly 16, 1940.

ELECTRIC RANGE UNIT CONTROL MECHANISM EMBODYIN MATERIAL 0F CHANGEABLECHARACTER Filed June 4, 1938 4 Sheets-Sheet 2 Mew? Jose DA .fiw

J. W. MYERS July 16, 1940.

ELECTRIC RANGE UNIT CONTROL MECHANISM EMBODYING MATERIAL OF CHANGEABLECHARACTER Filed June 4, 1958 4 Sheets-Sheet 5 4 SheetsSheet 4 y 1940- J.w. MYERS ELECTRIC RANGE UNIT CONTROL MECHANISM EMBODYIN MATERIAL 0FCHANGEABLE CHARACTER Filed June 4; 1958 Basis Cairo! Patented July 16,19.40

UNITED STATES 2,207,871 ELECTRIC RANGE UNIT CONTROL MECH- ANISMEMBODYING MATERIAL OF CHANGEABLE CHARACTER Joseph W. Myers,

Proctor & Schwartz,

Philadelphia, Pa., assignor to Incorporated, Philadelphia, Pa., acorporation of Pennsylvania Application June 4, 1938 Serial No. 211,899

12 Claim.

This invention relates to electric range cooking units and the like, andmore particularly to novel control means for effecting rapid heating ofsuch units to a desired temperature level.

The use of electric ranges and other electrical devices employingsurface cooking units or plates has been limited or retarded in the pastby the fact that such devices require appreciable time to attain theirnormal operating temperature and heat output and in' this respect arenot comparable in operation to gas ranges. It is well known thatelectric ranges have certain definite advantages over gas ranges, butsuch advantages have heretofore been generally outweighed by theinherent inability of electric range units to rise rapidly to theirnormal operating temperature.

It has been proposed heretofore to temporarily overload or increase theenergization of an electrical surface cooking plate unit by temporarilylowering the resistance of the unit to the supply voltage, and it hasbeen proposed to employ a thermostat directly associated with the plateso as to be effected by the temperature thereof and arranged to restorethe resistance of the heating unit to its normal value when the devicehas arrived at its normal operating temperature. It has also beenproposed to decrease the resistance of the heating unit either byproviding a normally short-circuited portion of the unit which isrendered effective by the thermostat when the unit has reached itsnormal operating temperature, or by providing a multi-section heatingunit and initially connecting the sections in parallel relation andarranging the thermostat to connect them in series relation when theunit has reached its normal operating temperature. While these proposalshave been improvements over the ordinary range unit, they have fallenshort of providing a practical and commercially satisfactory range unitor surface cooking plate.

A thermostat necessarily involves a moving part or parts and for thisreason it requires servicing from time to time. If it is located whereit is subject to accumulation of food substances, it is apt to bedeleteriously affected. A striking example of this is found in somebread toasters in which the thermostatic switch is not protected againstaccumulation of bread particles and such particles frequently becomelodged between the switch contacts, preventing them from closing. Themost convenient place to locate a thermostat for controlling a rangeunit or cooking plate is on the bottom surface of the plate, butexperience has shown that it is difficult in practice to so locate asufliciently sensitive mostat and, at the same time for servicing andinspection and properly protected from inevitable spillage of foodsubstances.

Furthermore, it is diflicult to manufacture an accurate high temperaturethermostat which will operate certainly at the highest safe temperatureto which it is desired to heat a range unit rapidly. Obviously, it isinstance that the thermostat operate certainly at the desiredtemperature, since a delay of a few seconds occasioned by the failure ofsome moving part to operate at the proper time might well provedestructive to the heating unit.

Thus, it is manifest that a control device, such as a thermostat, whichdepends for its accuracy of control upon a sensitive movable part orparts, has serious limitations in practical use, at least so far asconcerns the control of rapid heating of a range unit. The presentinvention eliminates the necessity of employing such a control device,and provides a novel control system employing a control device which isfree of sensitive movable parts and which may be located on the bottomsurface of the range unit or plate without deleterious effects fromspilled food substances or the like. Further, the control devicerequires less servicing, if any, than the usual thermostatic device.This device functions by virtue of a heat-changeable character of animmobile element rather than by the movement or deflection of a mobileelement as found in the conventional thermostatic device.

The principal object of the present invention, therefore; is to providea novel control mechanism for effecting rapid heating of an electricrange unit, which mechanism embodies a device or element having theproperty that a certain characteristic thereof is varied by heat. Thisdevice is used to control the rapid heating of the range unit, which maybe aptly termed flashing of the unit, to its normal high temperature.

Another object of the invention is to provide a control mechanism ofthis character wherein it is impossible to flash the unit eitherwillfully or accidentally to a temperature higher than the normal highoperating temperature level of the unit.

A further object of the invention is to provide a control mechanism ofthe stated character wherein an electrical element whose resistancevaries in response to heat is employed to control the flashingoperation.

A still further object of the invention is to provide a controlmechanism or the stated character wherein a magnetic element whosepermeability varies in response to heat is employed to control theflashing operation.

Other objects and features of the invention will appear hereinafter.

In the accompanying drawings:

Fig. 1 is a diagrammatic illustration of one form of control systemembodying the principles of the invention;

Figs. 2 and 3 are detail views illustrating different positions of thecontrol switch of Fig. 1;

Fig. 4 is a detail sectional view of the mechanical structure of thecontrol device employed in Fig. 5 is a bottom face vice;

Fig. 6 is an illustration of a control system embodying another form ofthe invention;

Fig. 7 is a view, partly in section and partly in elevation, of themechanical structure of the heating unit and associated parts;

Fig. 8 is a view taken along line 8-8 of Fig. 7 showing the parts indifferent position;

Fig. 9v is a fragmentary view showing more clearly the disposition ofthe parts in Fig. 8; and

Fig. 10 is a graph showing performance curves of the control elementsemployed in the illustrated embodiments.

view of the control de- Referring first to Fig. 1, there is shown aheat-' ing unit I which preferably comprises a plurality of sections,there being two sections in the unit illustrated. The heating unit isenergized from a conventional three-wire supply line 2 which providesdifierent operating voltages, as will be well understood. For example,the voltage between adjacent conductors of the supply line may be voltswhile the voltage between the two outer conductors may be 230 volts. Theoperation of the heating unit I is controlled by means of a controlswitch 3 which has a low" operating position and a "high operatingposition as shown in Figs. 2 and 3, respectively. This switch maycomprise a plurality of resilient contact arms adapted to engagestationary contacts, as illustrated. The movement of the switch arms maybe eflected by cams on a common rotatable shaft, so that rotation of theshaft to different angular positions will effect the settings of theswitch shown in Figs. 1 to 3. Since this general form of switchconstruction is well known, it is unnecessary to illustrate the detailsthereof and, for the sake of simplicity, only the contact elements andtheir different settings have been illustrated.

In order to effect flashing of the heating unit I to its normal hightemperature level, there is provided an electrically operable switchingde- ,vice or contactor 4 which functions to change the normal seriesconnection of the heating unit sections to a parallel connection, tothus increase the energization of the unit to above normal, as abovementioned. While it is preferred to employ a multi-section heating unitand to flash the unit by temporarily connecting its sections in parallelrelation, the invention is not thus limited and the heating unit may beof any other suitable form and may have any suitable means associatedtherewith for flashing the unit to its normal high temperature. Thecontactor 4 comprises a stationary magnetic pole piece 5, a magneticarmature core 6 associated with the pole piece, a cross-bar 1 carried bythe armature, and an energizing winding 8. The cross-bar 'I may beformed of insulating material and carries upper contact bridges 9 and I0and lower contact bridges H and I2. The contact bridges 3 and Ill areadapted to engage stationary contacts l3 and I4, respectively, while thelower contact bridges ll and I2 are adapted to engage stationarycontacts I5 and I6. Normally, the contactor armature is gravitationallydisposed in its lower position, as shown, with the lower contacts closedand the upper contacts open; but when the winding 8 is energized, thecontactor armature is moved to its upper position, opening the lowercontacts and closing the upper contacts.

It may now be noted that when the contactor 4 is in its normal positionand when the switch 3 is in its high position illustrated in Fig. 3, theheating unit sections are connected in series relation across the highvoltage conductors through acircuit which may be traced from theuppermost supply conductor, through contact arm 11, closed contacts l5,and the heating unit sections in series to the lowermost supplyconductor. The heating unit is thus energized for normalv highoperation. When the switch 3 is in its low" position, as shown in Fig.2, the heating unit sections are connected in a series relation acrossthe two lowermost supply conductors through a circuit which may betraced from the intermediate supply conductor through contact arm l8 andover the current path traced above to the lowermost supply conductor.The heating unit is thus connected for normal low operation.

By means of the mechanism to be described presently, the contactor 4 isactuated whenever the control switch 3 is thrown to its "high? position, unless the heating unit is already operating substantially at itsnormal high temperature level. Assuming that the switch 3 is in its highposition and the contactor 4 is moved to actuated position, it will beseen that the heating unit sections are then connected in parallelrelation across the high voltage supply conductors, through a circuitwhich may be traced from the uppermost supply conductor, through contactarm I1, closed contacts l3, through the heating unit sections inparallel and back to the lowermost supply conductor, one of the parallelbranch circuits including the left hand heating unit section, while theother branch circuit includes the right hand heating unit section andthe closed.

contacts l4. This connection of the heating unit effects rapid heatingthereof until the contactor 4 is deenergized to restore the normalseries connection of the heating unit sections for normal highoperation.

The purpose of the contacts l2 and IS on contactor 4 is to open one ofthe high voltage supply conductors during the flashing operation, tothus deenergize or prevent energization of any subsequent unit connectedto the said supply conductor, as represented by the unit l9. Thisdropping of a subsequent unit or load during the overenergization of aflashable unit prevents overloading of the apparatus as a whole.

The control mechanism for contactor 4 in the specific form of theinvention illustrated in Fig.

' 1 will now be described. A transformer T has its primary winding Pconnected as illustrated so as to be energizable by the contact arm 20of switch 3. It will be noted that the switch arm 20 is closed only whenthe switch 3 is in its "high position. One extremity of the secondarywinding S is connected to the anode a of a gridcontrolled rectifier tubeV, while the other extremity of winding S is connected through winding 8to the cathode c of tube V. 'The tube V is a grid-controlled rectifierof the cold cathode gas-filled type and this tube may be a so-calledgrid glow tube. A resistance R1 of high value is connected between theanode a and grid 0 of the tube as illustrated. To effect the controlaction contemplated by the invention, there is provided a resistor Rwhose resistance is varied by heat, as described hereinafter. Thisresistor is connected as illustrated between the lower extremity ofwinding S and the grid of tube V. In Fig. l, a conventional symbol hasbeen employed to represent the variable resistor R and this resistor isshown adjacent a portion of the heating unit I to indicate that it ispreferably arranged to receive heat from the heating unit.

In Figs. 4 and 5, there is illustrated a suitable physical embodiment ofthe resistor R in association with the heating unit. As shown in Fig. 4,the heating unit is preferably. of the type employing highly compressedinsulating material 2| completely embedding the wires 22 and having ametallic sheath comprising upper and lower metallic plates 23 and 24,respectively. Such a unit is capable of being flashed to its normaloperat-' ing temperature level in to 45 seconds by means of the systemillustrated. The resistor R of Fig. 1 comprises a device which ispreferably secured to the lower plate 24 as illustrated inFig. 4. Thisdevice comprises electrode disks 26 and 26 which are electricallyinsulated from plate 24 and from the securing bolt 21 by means ofinsulating washer 28 and 29. The electrode disks 25 and 26 are providedwith terminal lugs 30 and 3|, respectively, for electrical connection tothese disks. Between the disks 25 and 26, there is interposed a disk 32which is formed of a material whose electrical resistance is varied byheat. This element should normally have very high resistance to currentflow therethrough and its resistance-temperature characteristic shouldbe of such character in relation to resistor R1 and tube V that the tubeis normally unblocked but becomes blocked when the heating unit reachessubstantially its normal high temperature. A spring washer 21a may beemployed to maintain good contact between the several disks.

Certain materials exhibit a negative resistancetemperaturecharacteristic which changes rather critically within a certaintemperature range at about the normal high temperature of an electricalheating unit. For example, in Fig. 10 there is shown a curve Xrepresenting substantially the resistance-temperature characteristic ofceramic materials, such as porcelain. Moreover, such materials may bemade to perform electrically, in response to heat, in a desired mannerby simply varying the content of electrically conductive constituentsthereof, that is by adding varying quantities of electrically conductivematerial. In general, however, common forms oi ceramic materials willserve the purpose of the invention.

Referring to the curve X, it will be noted this curve variessubstantially exponentially or to a substantially greater degree thanlinearly over a temperature range including the normal high temperatureof a range unit. The critical control range or zone may be selected, asrepresented for example by the dot-and-dash vertical lines, and theapparatus of Fig. 1 may be designed accordingly to effect the operationnow to be described.

In considering the operation of the apparatus of Fig. 1, it should beborne in mind that a tube of the type shown is characterized in that acertain positive potential on the grid is necessary to start or fire thetube, after which the grid loses control, but the employment ofalternating current enables the grid to regain control and block thetube, when the grid potential falls below the said positive value, sincethe anode current passes through zero during each cycle.

When the control switch 3 is thrown to its .high position shown in Fig.3, the voltage across the secondary of transformer T, which should be ofsufllcient magnitude to start the tube, is applied between the cathodeand anode. Since the grid is connected to the anode through resistor R1,the grid is at substantially the same potential as the anode. Therefore,when the anode and grid go positive during the first cycle, the grid ischarged positively with respect to the cathode sufiiciently to start thetube, thus causing space current to flow in the tube and thus energizingthe winding 8 which is included 'in circuit with the tube. Thereafter,the grid loses control in the presence of the anode current flow, whichis a characteristic of tubes of this type. However, during each cycle ofthe alternating supply current, the anode current decreases to zero,thereby giving the grid an opportunity to regain control. The condenserC, which is shunted across the winding 8, becomes charged and maintainsa continuous and stabilized current supply, so that a relatively steadycurrent is applied to winding 6. This, together with the pole piece 5,prevents chatter of the contactor. The energization of contactor 4connects the heating unit sections in parallel relation across the highvoltage supply conductors,

as above described. The heating unit is, therefore, heated rapidly. Dueto the normal high resistance of the resistor R, the circuit throughthis resistor is substantially open-circuited, thus isolating the gridelectrically from the cathode.

When the temperature of the element 32 approaches the normal hightemperature of the heating unit, the resistance of this elementdecreases rather critically, as shown by curve X of Fig. 10, or in otherwords the conductivity of element 32 increases. When the resistance ofelement 32 decreases substantially to a predetermined value, theconductivity of this element causes the grid of tube V to more nearlyassume the potential of the lower end of winding S. In-

other words, the potential of the grid approaches that of the cathodeand reaches a value relative to'the cathode potential below the positivevalue required to start the tube. Consequently, when the anode currentdecreases to zero during the cycle, the grid regains control and blocksthe tube. This deenergizes the winding 6. The contactor 4 is thusdeenergized, restoring the normal series connection of the heating unitsections for normal high operation. During the normal high operation,the tube V remains blocked because the grid potential remains below thestarting value, so that the contactor 4 remains deenergized.

It will be apparent that by proper design of the apparatus, the tube maybe easily made'to cut-oil within the control zone indicated on curve X.As may be seen from curve X, the resistance of R decreases by about foreach 100 temperature rise. In the vicinity of the control zone, theratio of resistance change to temperature change is greater than two.Thus, for a temperature increase of 10%, the resistance of R decreasesmore than 20%. Such rate of decrease of R is ample to assure blocking ofthe tube V within a narrow temperature range, and therefore accurateoperation of the tube to control the flashing operation is readilyobtained, owing to the resistance-temperature characteristic of- Whenthe control switch 3 is thrown to its low position as shown in Fig. 2,the heating unit sec-. tions are connected in normal series relationacross the low voltage supply conductors and the heating unit is,therefore, energized normally to raise its temperature to the lowtemperature.

It may now be noted that the resistor R prevents overflashing of theheating unit, that is, flashing of the unit to a temperature in excessof its normal high temperature level. Since the element 32 is alwaysheat-conditioned according to the temperature of the heating unit, theresistance of this element decreases in the manner above describedwhenever the temperature of the heating unit approaches closely to thenormal high temperature level. Consequently, the tube V is alwaysblocked or cut-off whenever the temperature of the heating unit reachessubstantially. the normal high temperature level, and,

therefore, the flash contactor cannot be energized to over-flash theheating unit. It will be noted further that over-flashing of the heatingunit cannot be caused by any of the parts becoming defective. Forexample, if the tube V became defective, or if the transformer happenedto burn out, the contactor 4, if it is energized,

will be immediately deenergized, and it will be other words, if any partof the system becomes defective and ceases to operate, the heating unitmay be operated only in the normal manner, thus preventing burning outof the heating unit. Furthermore, since the tube V is conductive onlyduring the'flashing operation, its life is much greater than would bethe case if the tube. were operated during normal operation.

' The combination of the control operating by change of conductivity ofa material, the relay or flash contactor 4 having an energized positionfor flash and a deenergized position for normal,

and the load dropping contacts I2, I6 on the contactor, constitutes animportant featureof the invention. If it were possible for the flashoperation to continue due to some defective condition of the apparatus,the unit I would be burned out and the heating units subsequent to unitI, such as unit I9, would be inoperative due to the open contacts. I2,I6. But this cannot happen in the present device since any defectivecondition will cause the contactor 4 to return to normal, thus not onlyprotecting unit I but also rendering the subsequent units operative. Forexample, if the voltage fails in the control circuit, the contactor 4 isrestored from its flash position to its normal position.

Referring now to Figs. 6 to 9, there is shown an alternative form of theinvention, wherein the control of the flashing operation is eflected bymeans of a magnetic element whose permeability varies with temperaturesubstantially as shown by curve Y of Fig. 10. The heating unit la issimilar to that of Fig. 1 and is controlled in a similar manner by thecontactor 4a (shown in flash position) of the same general constructionas that employed in Fig. 1. In this instance, there is provided athermostatic control switch 33 which controls the heating unit onlyduring normal energization thereof. It will be noted from Fig. 6 thatthe switch 33 is in circuit with the heating unit only when thecontactor 4a is deenergized, and, therefore, the switch 33 isineffective to control the heating unit during flashing thereof.Consequently, the switch 33 cannot interrupt the flashing of the heatingunit. The switch 33 preferably takes the form illustrated in Fig. '1.Referring to that figure, there is provided a metallic casing 34, theupper poris seated upon a spring or springs 38 which urge the casingupward so as to maintain good contact with the cooking vessel. In theabsence of the cooking vessel, the annular shoulder 39 of the casingabuts against the cooking plate or heating unit so as to receive heatbyv direct thermal conduction therefrom. Within the casing 34 there isprovided a metallic supporting strip or plate 40 which is secureddirectly to thetop of the casing, and there is attached to thesupporting strip a thermostatic element 4| which is preferably composedof bimetallic material. It willbe seen, therefore, that the thermostaticelement receives heat principally by direct thermal conduction from thecooking vessel; and in the absence of the vessel, the thermostatic eleofthe abutment of the shoulder 39 therewith. ment receives heat by directthermal conduction from the heating unit or cooking plate by virtue Atits free end thermostatic element 4| pivotally carries a contact lever42, on one end of which there is mounted a contact 43 adapted to engagea stationary contact 44. A spring 45 serves to urge the contact lever 42in counterclockwise direction as viewed in Fig. 7, thus urging contact43 into engagement with contact 44. Thelatter' contact is mounted upon asupport 46 which is carried by the casing 34. The two contacts 43 and 44constitute the switch 33 which is connected electrically in the mannershown in Fig. 6.

The other end of lever .42 .is engageable by a cam 41 carried by'arotatable shaft 48 which in turn is journaled in support 49. By rotatingthe cam 41, the switch 33 may be opened manually, or the cam may bevariously positioned to adapt the thermostatic switch to maintain adesired at which the switch contacts are opened will depend upon thesetting of cam 41. If there is no cooking vessel on the heating unit,the thermostatic. element will respond to the temperature of the unitand will prevent it from being maintained above a certain temperaturecorresponding to the adjustment of cam 41. Thus, the thermostatic switchsubstantially prevents the heating unit or cooking plate from operatingbeyond a pertain temperature during normal energization of the heatingunit, and in addition, the switch controls the operation of the heatingunit in response to the temperature of the cooking vessel.

Referring again to Fig. 6, the energization of the flash contactor 4a iscontrolled by means of a switch 50 which in turn is controlled by themechanism now to be described. On the bot-' a flat strip which is formedof magnetic ma- I alloying of materials.

normally maintainthe'bar 52 in itsiowermost terial having thecharacteristic that its permeability is not significantly affected byheat until it is heated to a certain temperature and then itspermeability decreases abruptly. This strip may be formed of magneticiron, the permeability of which decreases non-linearly or exponentiallysubstantially asshown by curve Y of Fig. 10. This curve isrepresentative of the behavior of iron. In any case, however, a suitableperformance characteristic may be readily obtained by The strip 5| ismounted in thermalconductive relation with respect to the heating unitor cooking plate, so that the strip is always heat-conditioned accordingto thetemperature of the heating unit or cooking plate. In cooperativerelation with the strip 5|, there is a magnetic yoke 52 comprising apermanent magnet 53 and soft iron pole pieces 54 which are adapted toengage the strip 5|. Normally, the magnetic character of strip 5| willcause the magnetic yoke 52 to be held against strip 5| as in Figs. 6 and8, whenever the magnetic yoke is moved into engagement with the strip orinto close proximity thereto. It will be understood that the strip 5|normally furnishes a low reluctance path for the magnetic flux emanatingfrom the permanent magnet 53 and the associated pole pieces 54. When thepermeability of strip 5| decreases, however, it presents higherreluctance to the magnetic flux, and'when the permeability of strip 5|decreases sufilciently, it interrupts the magnetic attraction betweenthe yoke and strip 5|, thus interrupting the flashing operation, as willbe more clearly understood later.

The strip 5| being narrow and of relatively small areaand mass, does notabsorb any more heat from the heating unit than is necessary. Further,the slender pole pieces 54 absorb a minimum of heat and do not cause thepermanent magnet 53 to heat to an objectionable degree.

The magnetic yoke 52 is carried by a plate 55 having a depending guidepin 55 slidably seated in a guide 51 supported by a stationary bracket55. The plate 55 is carried by a bar 59 having projecting pins 50 -whichare seated in slots 5| of an adjacent bar 52. The bar 52' is carried bythe armature 53 54 which is energizable by means of the manual pushbutton switch 55.

The switch 59 comprises an upper contact arm or spring finger 55 and alower contact arm or spring finger 51 having an aperture 58 throughwhich an insulating pin 69 may project. The pin 59 is carried by alateral extension 10 of bar 52. A spring H is arranged as illustrated;to

position and to resist upward movement of this bar. On the bar 59 thereis provided a lateral extension 12 carrying an insulating stop 73against which the end of the contact arm, 51 normally abuts, as shown inFig. 7. The resilience of contact arm 51 normally maintains this armagainst stop 13 in switch-opening position, as shown in Fig. 7.

Whenever the switch 55 is closed, the solenoid or magnet 54 isenergized, thereby moving bar 62 upward. When the bar 52 has movedupward sufliciently to take up the lost motion of the pins and slots, itcarries the bar 59 upwards, thus causing the magnetic yoke to move intoengagement with strip 5|, as shown in Fig. 8. The pin 59 passes throughthe opening 58 and engages of an electromagnet or solenoid arm 55deflecting this arm upward (see Fig. 9)

to prevent closing of the switch until bar 52 returns to its lowerposition. When switch 55 is released, the magnet or solenoid isdeenergized, causing bar 52 to move downward under the influence ofspring 1|, permitting arm 55 to return to its lower position, as shownin Fig. 6. The bar 59, however, is maintained in its upper position bythe magnetic attraction between the magnetic yoke and the strip 5|.Therefore, the contact arm 51 is maintained in switch-closing position,as shown in Fig. 6, and the switch 59 is closed. The contactor 4a is,therefore, energized to effect flashing of the heating unit, aspreviously described. When the heating unit has reached substantiallythe normal high temperature level, the permeability of strip 5|decreases abruptly, thereby permitting bar 59 to move downwardgravitationally to effect opening of switch 50 by virtue of the loweringof stop 13. The consequent deenergization of contactor 4a restores thenormal energization of the heating unit.

In Fig. 10, the relatively narrow control range or zone is indicated oncurve Y by the dot-anddash vertical lines. It will be apparent from thecurve that accurate operation is readily obtained within the relativelynarrow range of critical change of the permeability of the magneticmaterial. In the vicinity of the control zone, the ratio of percentagedecrease of the permeability to the percentage increase of temperatureis reater than two.

It may now be noted that the magnetic strip 5| controls the flashingoperation and prevents. overflashing of the heating unit. It will benoted further that the pin 59 prevents over-flashing of the heating unitby the operators holding the switch 55 closed. Since the pin 59 preventsclosing of switch 55 until bar'52 has dropped to its lower position, theflash contactor 4a will not be energized until the push button switch 55has been released. Therefore, it is impossible for the operator toover-flash the heating unit by holding the switch 55 in closed position,whether this action be willful or unintentionah It will be seen that ineach of the embodiments disclosed, the control is effected by animmobile solid element having the property that a certain character ofthe element varies negatively and exponentially to effect the controlaction, and the element is continuously heat-conditioned in accordancewith the temperature of the heating unit. In other words, thetemperature of. the control element substantially tracks with thetemperature of the heating unit. It is not necessary that thetemperatures be the same but only that one be a function of the otherand that the control action take place when the heating unit reachessubstantially its normal high temperaing of the heating unit, providedthat the control temperature 01'. the control element is reached whenthe heating unit reaches substantially its normal high temperaturelevel.

Both of the disclosed embodiments are free of any sensitive movablecontrol elements, such as employed in a thermostat, and therefore theyare not subject to the limitations of the thermostat above noted. Ineach instance the control is eflected by a physical change that isincapable of accidental or intentional suspension. In the firstembodiment, the conductivity of the control material increasesprogressively by a large factor if the control operation does not takeplace at the temperature desired, and therefore the control actionincreases exponentially until the flash is terminated. In the secondembodiment. there can be no suspension of the change in permeability ofthe'magnetic material and therefore the termination of the flashoperation is bound to occur at or about the desired temperature.

Although two specific embodiments of the invention have been illustratedand described for the purpose of disclosure, it will be apparent thatother forms of the invention are possible. In its broad scope theinvention contemplates the control of. a flashing mechanism by means ofan ele-,'v

. conditioned in substantial accordance with the temperature of theheating unit, electricallyoperable means for increasing the energizationof said unit to above normal, to thereby cause said unit to heatrapidly, a switch for controlling the energization oi. said last means,a magnetic member arranged cooperatively with said element but normallydisengaged therefrom, pole pieces extending from said magnetic memberand having portions of small area for engaging said element. means formoving said magnetic member to bring said pole pieces into magneticattracted engagement with said element, the said small area portions ofsaid pole pieces reducing the transfer of heat from said element to aminimum, and a switch-actuating member attached to said magnetic memberand adapted to close said switch, whereby said switch is maintainedclosed until said magnetic member is released by said element when thepermeability of the latter decreases substantially. I

2. In an electrical cooking apparatus, an electrical heating unit, meansfor energizing said unit, a magnetic element whose permeabilitydecreases critically from its normal value when the' element is heatedto a certain temperature, said element being arranged to beheat-conditioned in substantial accordance with the temperature of theheating unit, electrically-operable means for increasing theenergization of said unit to above normal, to thereby cause said unit toheat rapidly, a switch for controlling the energization of said lastmeans, a magnetic member arranged cooperatively with said element butnormally disengaged therefrom, manually operable means for moving saidmagnetic member into magnetic attracted relation with said element, aswitch-actuating member attached to said magnetic member and adapted toclose said switch, and means for preventing closure of said switch untilsaid manual means is released.

3. In an electrical cooking apparatus, an electrical heating unit, meansfor energizing said unit, a magnetic element whose permeabilitydecreases critically from its normal value when the element is heated toa certain temperature, said element being arranged to beheat-conditioned in substantial accordance with the temperature of theheating unit, electrically-operable means for increasing theenergization of said unit to above normal, to thereby cause said unit toheat rapidly; a switch for controlling the energization 01 said lastmeans, a magnetic member arranged cooperatively with said element butnormally disengaged therefrom, manually energizableelectrically-operable means for moving said magnetic member intomagnetic attracted relation with said element, a switch-actuating memberattached to said magnetic member and adapted to close said switch, andmeans for preventing closure of said switch until said manuallyenergizable means is deenergized.

4. In an electrical cooking apparatus, an elec-' trical heating unit,means for energizing said unit, thermal-responsive means engageable by acooking vessel placed on said unit for controlling the operation of saidunit during normal energization thereof, means for increasing theenergization of said unit to above normal, to cause said unit to heatrapidly, and means including an immobile element of heat-changeablecharacter arranged to be heat-conditioned in substantial accordance withthe temperature of. said unit for restoring the normal energization ofsaid unit when the unit is heated substantially to its normal hightemperature level.

5. In electric heating apparatus, an electric heating unit, at least oneother electric heating unit, a supply line for supplying electricalenergy to said units, an electric switch for each unit manually operableto render either or both of said units active at will, another electricswitch operable from one position to another to increase I theenergization of said first unit above a level normally available throughthe contacts of said first switch, to thereby cause said first unit toheat rapidly to a predetermined temperature level, means controlled bysaid other switch, for opening the supply line ahead oi. said other unitbefore the energization of said first uniths increased above normal, tothereby prevent energization of said other unit during abnormalenergization of said first unit, and means including an immobileheat-changeable solid element for automatically efl'ecting returnoperation of said other switch when said first unit reaches saidtemperature level, to thereby restore normal energization of said firstunit and to close the supply line to said other unit, whereby prolongedoverenergization of said first unit is prevented and prompt restorationof the energy supply to said other unit is assured.

' 6. In electric heating apparatus, an electric heating unit, at leastone other electric heating unit, a supply line for supplying electricalenergy to said units, an electric switch for each unit ahead of saidother unit before the energization of said first unit is increased abovenormal, to thereby prevent energization of said other unit duringabnormal energization of said first unit, and means including animmobile heat-changeable solid element for deenergizing said energizingcircuit and thus efi'ect automatic return operation of saidlast-mentioned switch when said first unit reaches said temperaturelevel, to thereby restore normal energization of said first unit and toclose the supply line to said other unit, whereby prolongedoverenergization of said first unit is prevented and prompt restorationof the energy supply to said other unit is assured.

7. In electric heating apparatus, an electric heating unit, at least oneother electric heating unit, a supply line for supplying electricalenergy to said units, an electric switch for each unit manually operableto render either or both of said units active at will, anelectrically-operable switch operable from a normal deenergized positionto an energized position to increase the energization of said first unitabove a level normally available through the contacts of said firstswitch, to thereby cause said first unit to heat rapidly to apredetermined temperature level, a manually-closable energizing circuitfor said last-mentioned switch, contacts at the normal position of saidlast-mentioned switch for opening the supply line ahead of said otherunit before the energization of said first unit is increased abovenormal, to thereby prevent energization of said other unit duringabnormal energization of said first unit, a grid-controlled spacedischarge device for controlling said energizing circuit, an electricalresistance element of heat-variable character arranged to beheat-conditioned in substantial accordance with the temperature of saidfirst unit, and a grid-control circuit including said resistance elementfor controlling the operation of said space discharge device so as todeenergize said energizing circuit when said first unit reaches saidtemperature level, to thereby efi'ect automatic return of saidlast-mentioned switch to normal position and thus restore normalenergization of said first unit and reclosure of the supply line to saidother unit.

8. In an electrical cooking apparatus, an electrical heating unit, meansfor energizing said unit, an electrical resistance element orheatvariable character arranged to be heat-conditioned in substantialaccordance with the temperature oi the heating unit,electrically-operable means for increasing the energization of said unitto above normal, to thereby cause said unit to heat rapidly, anenergizing circuit for said last-mentionedmeans, a space dischargedevice comprising a cold cathode,'anode and control electrode, means forincluding the-space current path or said device serially in saidenergizing circuit, and a circuit for said control electrode includingsaid resistance element and arranged to render said space dischargedevice substantially non-conducting when said heating unit reaches asubstantially predetermined temperature, to thereby restore the normalenergization of said unit. a

9. In an electrical cooking apparatus, an electrical heating unit, meansfor energizing said unit, an electrical resistance element ofheatvariable character arranged to be heat-conditioned in substantialaccordance with the temperature of the heating unit,electrically-operable means for increasing the energization of said unitto above normal, to thereby cause said unit to heat rapidly, anenergizing circuit for said last-mentioned means, a grid-controlledgasfilled space discharge device comprising a cathode, anode and controlelectrode, means for including the space current path of said deviceserially in said energizing circuit, means for normally maintaining saidcontrol electrode at a potential suificient to start said device butineflective to stop the same and means including said resistance elementfor applying to said control electrode a potential efl'ective to rendersaid space discharge device substantially non-conducting when saidheating unit reaches a substantially predetermined temperature, tothereby restore the normal energization of said unit.

10. In an electrical cooking apparatus, a cooking unit comprising ametallic casing, means for energizing said unit, a thin body ofheat-variable resistance material, means for clamping said body inthermal conducting relation with said casing, electrically-operablemeans for increasing the energization of said unit to above normal, tothereby effect rapid heating of said unit, an energizing circuit forsaid last-mentioned means, and means including said body of resistancematerial for deenergizing said circuit when said unit reachessubstantially a predetermined temperature, thereby restoring normalenergization of the unit.

11. In an electric cooking apparatus, an electrical heating unit, meansfor energizing said unit, thermal-responsive means engageable by acooking vessel placed on said unit for controlling the operation of saidunit during normal energization thereof, means for increasing theenergization of said unit to above normal, to cause said unit to heatrapidly, and means including an electrical resistance element ofheat-changeable character arranged to be heat-conditioned in substantialaccordance with the temperature of said unit for restoring the normalenergization of said unit when the unit is heated substantially to itsnormal high temperature level.

12. In an electrical cooking apparatus, an electrical heating unit,means for energizing said unit, thermal-responsive means engagea-ble bya cooking vessel placed on said unit for controlling the operation ofsaid unit during normal energization thereof, means for increasing theenergization of said unit to above normal, to cause said unit to heatrapidly, and means including a magnetic element of heat-changeablepermeability arranged to be heat-conditioned in substantial accordancewith the temperature of said unit for restoring the normal energizationof said unit when the unit is heated substantially to its normal hightemperature level.

JOSEPH W. MYERS.

